update hashtree_eqc. get it passing on eqc 1.29.1

* get rid of make_treevars since the {call, erlang,'*', _} was the
cause of unknown precondition failures and the same generation can be
done without it
* remove initial_state function and just pass the rest of the
initial state to commands/2
* use pretty_commands/4 so failures are easier to track down